Enable serving static files from the root instead of just "public".

I'd like to be able to serve static files from the root of an app instead of just a `public` directory.  This is useful when using pow to serve static sites - an extra directory level then isn't necessary.

I've enabled this based on the presence of a `.powstatic` file in the root - if it's there the static handler is configured to serve from the root, otherwise the `public` directory is used.